One reason to use an applet to do this task, or any other task, would be that it's written in Java. Of course if you have a question about why this specific requirement exists,
you should ask the person who made up the requirement. Asking random strangers on the Internet why the requirement exists wouldn't make much sense.
I don't understand your performance question. Are you asking for a comparison between something written as an applet and the same thing written in Javascript? You aren't going to get more than a wild guess for a question that unspecific. My wild guess would be that most of the processing would be done at the server -- your idea that the task can be done in Javascript leads me to believe that there is server-side processing involved -- and in that case the client-side processing wouldn't make much difference. But that could be completely wrong.